The invention relates to a cathode-ray tube and, more particularly, to a low power cathode assembly for such a tube in which the thermal losses due to heat conduction along the cathode sleeve as well as radiation and conduction losses from the legs of the cathode heater are reduced.
U.S. Pat. No. 3,772,554, issued to R. H. Hughes on Nov. 13, 1974, discloses a conventional inline electron gun having three cathode assemblies and a plurality of spaced electrodes individually attached to a pair of glass support rods. The beam forming region comprising the cathode assemblies, the control grid (G1) electrode and the screen grid (G2) electrode is shown in FIG. 1. A portion of the main electron lens, including the focusing electrode (G3) is also shown in FIG. 1. The conventional cathode assemblies disclosed in the Hughes patent typically operate at about 1.3 to 1.6 watts of input power per cathode assembly. This level of power consumption causes a great deal of heat to be generated resulting in excessive longitudinal motion between adjacent electrodes in the beam forming region. Typically, the cathode-to-G1 spacing varies as much as about 0.08 mm (3 mils), and changes of about 0.025 mm (1 mil) typically occur between the G1 and G2 electrodes.
U.S. Pat. No. 4,298,818, issued to H. E. McCandless on Nov. 3, 1981, discloses an improved electron gun having a unitized cathode-grid subassembly comprising three cathode support members and two spaced successive electrodes (the G1 and G2 electrodes) individually attached to a single ceramic member, which is the sole supporting interconnection for the elements of the beam forming region. The cathode assemblies described in the McCandless patent are conventional and operate at about 1.3 to 1.6 watts of input power per cathode. Despite the unitized construction disclosed in the McCandless patent, which improves alignment between adjacent electrodes, excessive longitudinal motion occurs between the cathode assemblies and the G1 electrode and between the G1 and G2 electrodes because of the amount of heat produced by the conventional cathode assemblies. The McCandless electron gun structure is shown in FIG. 2.
U.S. Pat. No. 4,370,588, issued to K. Takahashi et al. on Jan. 25, 1983, discloses, in FIG. 3 thereof, a low power cathode assembly of complex construction. In the patented cathode assembly, a first cylindrical reflective member, which has a disklike metal substrate thrusted and fixed in the top opening portion thereof, surrounds the upper portion of a cathode sleeve. The cathode sleeve and the first cylindrical reflective member are fixed to each other by welding or the like. The cathode sleeve is also fixed to a peripheral edge of a second cylindrical reflective member by means of three support straps, which are welded to the bottom end of the cathode sleeve at intervals of 120.degree. so that the cathode sleeve may be coaxial with the second cylindrical reflective member. The exposed exterior surface of the cathode sleeve is blackened to increase radiation therefrom. However, by disposing the first cylindrical reflective member around the top portion of the cathode sleeve, heat from the top portion of the cathode sleeve is reflected by the first cylindrical reflective member to reduce radiation to the outside. The patent discloses that heat radiated from the blackened surface of the cathode sleeve is equivalent to heat radiated from the surface of a non-conductive material. The amount of heat radiation from the blackened surface is substantially uniform for radiation at an angle exceeding 30.degree. to the radiation surface, but decreases drastically below 30.degree.. Thus, by properly locating the first and second cylindrical reflective members, most of the heat radiated from the blackened cathode sleeve is not radiated to the outside, so that a power saving cathode assembly can be obtained. The cathode assembly described in the Takahashi et al. patent requires a minimum of seven parts (not including a cathode heater) which must be welded together. Such a labor intensive structure is costly and complex. The need thus exists for a simple, low cost, low power cathode assembly.
U.S. patent application, Ser. No. 559,370, filed on Dec. 8, 1983, by S. T. Opresko, discloses a low power cathode assembly including a one-piece cathode sleeve closed at one end and having an integral cap. The cathode sleeve has a longitudinally extending first portion with an outside diameter of about 1.47 to about 1.50 mm that conforms closely to the heater for reducing the power requirements thereof. The cathode sleeve includes at least one other longitudinally extending portion which has a diameter greater than the diameter of the first portion. The first portion and the other portion of the cathode sleeve are connected by a transition region inclined at an obtuse angle to the longitudinally extending first portion of the sleeve. A plurality of openings are formed in the transition region of the sleeve to provide a heat dam to restrict the conduction of heat along the sleeve and to limit the radiative heat loss through the openings from the heater legs disposed within the cathode sleeve. The overall length of the cathode sleeve is disclosed to be 8.76 mm so that the legs of the heater must be sufficiently long to extend beyond the open end of the cathode sleeve for electrical connection to the heater straps. It has been found that the total heat loss of the heater legs is about 20 percent of the input power, thereby decreasing the efficiency of the cathode assemblies and raising the minimum power requirements for a three-cathode structure by about 0.50 watts.